The invention relates to silicone-based impression compositions with improved dimensional stability. The invention relates in particular to dental A-silicone impression compositions, the dimensional stability of which was increased by adding small amounts of polyalkylene oxides and/or their derivatives with an average molar mass of Mw greater than 3000.
In dentistry, dentures are made using models. To this end, a three-dimensional negative of the jaw situation is initially prepared by means of an impression composition, with, for example, the plastic, not yet set, impression composition being inserted into the patient""s mouth on an impression cup and setting there. It is also possible to initially spray the points of the jaw which are to be modelled particularly accurately, with impression composition from a so-called elastomer syringe, and only then to introduce a cup filled with impression composition into the patient""s mouth for the final cast. The impression composition ultimately sets to a hard or elastic composition which, following removal, represents the negative mould mentioned at the outset.
The most varied material classes are used as elastic impression materials, for example polyethers and silicones which are crosslinked by a chemical reaction to form the elastomer. Condensation and addition-crosslinking systems can be obtained with the silicones. The condensation-crosslinking silicones (C-silicones) crosslink as a rule through titanium- or tin-catalyzed reaction of the hydroxy-terminated polysiloxane with silicon alkoxy compounds accompanied by splitting off of an alcohol (condensation). In the case of the addition-crosslinking silicones (A-silicones), the crosslinking takes place as a rule through platinum-catalyzed reaction of unsaturated hydrocarbon terminal groups of the polysiloxane with Sixe2x80x94H groups of a hydrogen polysiloxane (hydrosilylation, addition). A-silicone impression compositions are known for example from EP-A-0 162 211, DE-A-40 31 759 and EP-A-0 166 107 as well as from the specialist literature, for example R. G. Graig, Restaurative Dental Materials, The C. V. Moosbe-Comp., St. Louis, 1980, p. 195ff.
The silicone compositions are usually supplied as a two-component systemxe2x80x94in the form of separately packed base and catalyst pastesxe2x80x94the components being automatically or manually dosed in a specific mixing ratio before use, mixed and then introduced into the patient""s mouth. They then cure there within a preset period of a few minutes.
In general, the base pastes are mostly composed of vinyl-terminated silicone oil, filler and crosslinkers and the catalyst pastes of vinyl-terminated silicone oil, filler and catalyst.
A disadvantage of many known silicone compositions is that they have a relatively low dimensional stability in the non-cured state. Attempts are made to deal with this disadvantage through various possibilities:
High degree of fill of the pastes with coarse inorganic fillers
Addition of pyrogenic silicic acids or of diatomaceous earth (WO-96/32088)
Addition of waxes (DE-A-195 17 962)
A disadvantage of these solutions is however that, on the one hand, poor flowability or insufficient precision of drawing, caused by the filler itself, is ascertained and, on the other hand, the additions tend to bleed, as they are added in not inconsiderable quantities, in the case of DE-A-195 17 962 of up to 40 wt.-%. A decrease in the Shore hardness and a deterioration in cuttability as well as storage stability have also been observed.
Furthermore, a practical disadvantage is that pastes with a high filler content or viscous additives such as waxes, can be ejected from their containers only with difficulty. Under certain conditions, it is no longer possible for the dentist or his assistant to dispense the pastes with one hand. Electrically-powered mixing apparatuses can cause damage if the ejection forces increase too greatly.
As high a dimensional stability as possible is necessary for example with bite registrations. A high dimensional stability is also desirable in the case of impression compositions for all other indications, in order that the non-set composition does not flow out of the impression cup into the patient""s pharynx upon introduction into the patient""s mouth and thus cause retching or in order that, upon spraying of individual jaw parts, the compositions do not drip due to gravity in the case of the upper jaw or flow away in the case of the lower jaw, and expose the prepared point in the jaw where particular precision is required. It is therefore necessary to find possibilities for increasing the dimensional stability of impression compositions without the previously described disadvantages of high ejection forces and bleeding being observed.
Surprisingly, it was found that additions of 0.001 to 1.0 wt.-% of polyalkylene oxides and/or their derivatives with an average molar mass of Mw greater than 3000 to A-silicone impression compositions permanently increase the dimensional stability of these compositions without adversely affecting the ejectability and Shore hardness.
The impression material according to the invention contains the following components:
(a) organopolysiloxanes with at least two unsaturated groups in the molecule,
(b) organohydrogen polysiloxanes with at least three Sixe2x80x94H groups in the molecule,
(c) optionally, organopolysiloxanes without reactive groups,
(d) platinum catalyst,
(e) optionally, hydrophilizing agent,
(f) filler,
(g) optionally, further customary additives, auxiliaries and dyes,
(h) polyalkylene oxides and/or their derivatives with an average molar mass of Mw greater than 3000 at the rate of 0.001 to 1.0 wt.-% relative to the total mass of the cured material.